非修改性序列算法之count和count

count主要用来统计容器内元素个数，其格式有两种：

第一种，第三个参数为元素值

template<class _InIt, class _Ty> inline typename iterator_traits<_InIt>::difference_type count(_InIt _First, _InIt _Last, const _Ty& _Val)

第二种，第三个参数为条件函数：

template<class _InIt, class _Pr> inline typename iterator_traits<_InIt>::difference_type count_if(_InIt _First, _InIt _Last, _Pr _Pred)

程序实例：

#include<iostream> #include<functional> #include<algorithm> #include<vector> template<typename T> int PushNum(T &vec, int first, int last) { int ret = 0; if (first > last) { ret = -1; cout << "function PushNum first > last error:" << ret << endl; } while (first <= last) { vec.push_back(first); first++; } return ret; } template<typename T> void print(T &ele) { cout << ele << " "; } bool isEven(int &ele) { return ele % 2 == 0; } using namespace std; int main(){ vector<int> vec; PushNum(vec, 1, 10); for_each(vec.begin(), vec.end(), print<int>); cout << endl; int count1 = count(vec.begin(), vec.end(), 5); int count2 = count_if(vec.begin(), vec.end(), isEven); int count3 = count_if(vec.begin(), vec.end(), bind2nd(greater<int>(), 2)); cout << count1 << endl; cout << count2 << endl; cout << count3 << endl; system("pause"); return 0; }

输出结果：

第一个count是求元素值等于5的个数，第二个count是求满足偶数的个数，第三个count是求大于2的个数。